Binary Phase Diagrams

One of the most important part of physical metallurgy is binary phase diagrams.

Binary phase diagrams define the regions of stability of the phases that can occur in an alloy system of two components, under the condition of constant pressure.

That is why we will use condensed Gibb's phase rule in all binary systems ( i.e. P + F = C + 1 ).

Co-ordinates are temperature along the ordinate (X-axis) and composition along the abcissa (Y-axis). Basics are covered in previous post so do check it out. ( Read )

Fig.1

Pure components solidifies at a single temperature, while alloys in the isomorphous system solidifies over a range of temperature. Now this range changes with change in composition of alloy system.

Binary phase diagrams are plotted from multiple cooling curves of alloy systems for different compositions.






(1) Isomorphous System

In this system both components are completely soluble in both liquid and solid state. So there will be only two phases ( i.e. solid & liquid ).

A binary isomorphous diagram of components A & B, plotted from the cooling curves of different alloy compositions is shown in the Fig.2

Fig.2

T(A) = Melting/Freezing point of component A
T(B) = Melting/Freezing point of component B
Liquidus line = Upper line connecting all the points where the freezing begins.
Solidus line = Lower line connecting all the points where the freezing ends.

Area above the liquidus line is a single-phase field consisting of homogeneous liquid-solution and the area below the solidus line is a single-phase field consisting of homogeneous solid-solution. And between both lines is a two-phase field. Any alloy in this region will contain a mixture of liquid and solid solutions. The phase equilibrium across the two-phase field is called conjugate phases.


Miscible Solids 

Many system are comprised of components having the same crystal structure, and the components of some of these systems are completely miscible (soluble in each other) in the solid form, thus forming a continues solid solution.

When this occur in binary phase system, the phase diagram usually has the general appearance of that of the Fig.2 shown above.

Fig.3

If the solidus and liquidus lines meet tangentially at some point, a maxima as per Fig.3(a) or minima as per Fig.3(b) is produced in two-phase filed.

Fig.4

It is also possible to have a gap in miscibility in a single-phase field; as shown in Fig.4.

Region of  α1+α2  is called miscibility gap. Above point Tc, both α1 and α2 becomes indistinguishable.

Boundary line of miscibility gap is known as solvus line. It indicates the limit of solubility of component B in A and A in B, respectively.




(2) Eutectic Reaction

If miscibility gap in solid region as shown in fig.4 is expanded so that it touches the solidus line at some point, as shown in fig.5(a), complete miscibility of the components is lost.

Fig.5

Instead of a single solid phase, the diagram now shows two separate solid terminal phases ( i.e. α and β ), which are in three-phase equilibrium with the liquid at point P, an invariant point that occurred by coincidence.

Then, if this two-phase field in the solid region is even further widened so that the solvus line no longer touch at the invariant point, the diagram passes through a series of configurations, finally taking on the more familiar shape shown in Fig.5(b).

The three-phase reaction then takes place at invariant point E, where a liquid phase freezes into a mixture of two solid phase, is called eutectic reaction ( Greek word for "easily melted" ).

The alloy that corresponds to the eutectic composition is called eutectic alloy. Alloy having a composition to left of eutectic point is called hypo-eutectic alloy and that having a composition of right is called hyper-eutectic alloy.

For the cases where the two components have different crystal structure, the liquidus and solidus curves for each of the terminal phases ( Fig.5(c) ) resemble those for the situation of complete miscibility between system components shown in Fig.2.